1. Field of the Invention
The invention relates to a heating device of the light irradiation type in which a semiconductor wafer (hereinafter called a wafer) is quickly heated for layer formation, diffusion, annealing or for similar purposes, kept at a high temperature, and then quickly cooled.
2. Description of Related Art
Heat treatment of the light irradiation type in the production of semiconductors, such as layer formation, diffusion, annealing or the like, is carried out in a wide range. In each of these treatments, a wafer is heated to a high temperature. If a heating device of the light irradiation type is used for this heat treatment, the wafer can be quickly heated and its temperature increased up to at least 1000xc2x0 C. in a few seconds to a few dozen seconds. When light irradiation is stopped, it can be quickly cooled. But, if a nonuniform temperature distribution occurs in the wafer when it is heated, a phenomenon called slip arises in the wafer, i.e., a dislocation fault. Here, there is the danger of formation of scrap. Therefore, it is necessary in the heat treatment of the wafer using a heating device of the light irradiation type to carry out heating, keeping at a high temperature and cooling such that the temperature distribution of the wafer is made uniform.
Furthermore, in the case of heating for layer formation on the wafer, the latter must be heated with a high degree of uniformity of illuminance within the surface in order to form a layer with a uniform thickness.
FIG. 9 shows a conventional optical heating device in cross section. Here, in a light source part 1, there are filament lamps L2 to L10, countersunk, with circular arc tubes in which a filament for emission is located. FIG. 10 shows one arc tube. In the middle of the light source part is a lamp L1 with a unilateral hermetic seal (single end type). In the chamber 2, there is a holding frame 3 on which a workpiece to be subjected to heat treatment, such as a semiconductor wafer W or the like, is placed. The lamps L2 to L10 and the chambers 2 are separated from one another, for example, by a quartz window 4 or the like.
FIG. 11 is a schematic in which the light source part is viewed from the side of the workpiece (wafer). Here, there are several lamps with different annular diameters arranged concentrically relative to one another.
The lamps used here can also be made circular by combining two, or more than two, arc-shaped arc tubes with one another, as is shown in FIG. 12. FIG. 13 is a schematic in which the light source part in which lamps with arc-shaped arc tubes are arranged combined with one another is viewed from the side of the workpiece (wafer). In the middle area of the light source part, i.e., in the middle of the concentric circles of the circular or arc-shaped lamps (hereinafter both types are called xe2x80x9ccircular lampsxe2x80x9d), there is no lamp. If this state remains unchanged, the illuminance of the light with which the middle area of the workpiece is irradiated is reduced. Therefore, as shown in FIG. 9, there is a lamp L1 with a unilateral hermetic seal (single end type) in essentially the center area, and thus, the reduction of illuminance is equalized.
The irradiance of the single end lamp L1 located in the middle must be the same as the irradiance of the circular lamp L2 and the like located in the vicinity. The lamp of the single end type, however, has a smaller diameter filament than the circular lamps. Therefore, if the same wattage is supplied in order to make the irradiance of the single end lamp and that of the circular lamps the same, in the single end lamp, the filament burns through earlier than in the circular lamps. That is, in the lamp with the unilateral hermetic seal, the service life is shorter than in the circular lamps. The reason why the filament diameter of the lamp with the unilateral hermetic seal differs from the filament diameter of the circular lamps is the following:
As is shown in FIG. 14(a), the single end lamp has an arrangement in which a filament 21 is bent within a bulb 20. The diameter of the filament 21 is reduced to prevent the bent filament 21 from touching the windings and thus being short circuited. In the case of a circular lamp, as shown above in FIGS. 10 and 12, the filament is not bent. Therefore, as shown in FIG. 14(b), the diameter of the filament can be increased. The lamp service life (duration of illumination until the filament burns through) in the single end lamp is, for example, roughly 500 hours, while the service life of the circular lamp is roughly 2000 hours.
Since the service life of the single end lamp L1 which is located in the middle area is shorter than the service life of the other circular lamps, for replacement of the single end lamp L1, the device must be frequently stopped (roughly every 500 hours). To prevent frequent stopping of the device, it would be better not to use the single end lamp any longer. If this lamp is eliminated, however, the illuminance of the middle area on the irradiated surface is reduced, by which the distribution of illuminance is degraded and by which uniform heating of the entire surface of the wafer can no longer be performed.
The invention was devised to eliminate the aforementioned disadvantages in the prior art. Thus, a primary object of the invention is to devise a heating device of the light irradiation type in which uniform illuminance can be obtained without placing a single end lamp in the middle of the light source part.
The object is achieved as claimed in the invention as follows:
(1) In the above described heating device of the light irradiation type, in the middle area of the light source part, instead of a single end lamp, there is a reflector which is convex in the workpiece direction and which reflects the light from a filament lamp which is located on the innermost side in the direction to the middle area of the workpiece. In this way the light which is emitted from the innermost circular lamp can be reflected by the above described mirror in the direction to the middle area of the workpiece and the reduction in illuminance on the workpiece surface can be equalized.
(2) The above described reflector is formed preferably conically. In this way, the light from the lamps can be uniformly emitted onto the workpiece surface.
(3) Preferably d less than 2L, where L is the diameter of the innermost filament lamp and d is the distance between the middle of this filament lamp and the workpiece.
By virtue of the above described measure that d less than 2L, the action can be obtained that the illuminance is especially well equalized by the conical mirror. Especially, by the measure that the temperature of the middle area of the workpiece can approach the desired temperature.
The invention is explained in detail below using several embodiments shown in the drawings.